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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Tue 03, 2012 5:32 pm 
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You might be better off with the ECS 1.2288 MHz crystal and changing the 330 uH to 220 uH.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Tue 03, 2012 9:32 pm 
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Hi Sandy, I use a crystal at 1240kHz. That part of the band is quiet. 980 and 730 are flame throwers around here.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 1:09 am 
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35Z5 wrote:
Here's something to make tuning easier, install LED in B+ feed to LM386... Led will be brightest when antenna tuning is peaked(which is opposite the tube transmitters that we dip current)... Causes no issue but does drop approx 1.5v depending on color of diode, for battery operation a AA in series with your 9v is a good solution...

Another option is install the LED in place of the 150uh coil... This also works fine but due to the output of the LM386 being approx half of input voltage, B+needs to be raised closer to 12V...

Additionally unless one is using a very high impedance source, the input resistors can be as low as 1K and the gain pot 5K... That's what being used on the breadboard, performs just as well if not better than the higher values...

Image

Tom


Would it be possible to use a panel mount milliammeter in place of the LED if one is not concerned about enclosure size?


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 1:34 am 
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Got er done. Sounds good. I was surprised at how touchy the adjustment was on the trimmer but I was finally able to get it pretty well tweaked. However, there's a slight hum and I can't seem to pin it down. Here's a pic of my less than artistic creation. Power coming in from a wall wart at the back. I did add the extra cap, C2. Audio input on the left and antenna connection on the right. Anything jump out at anyone?
Attachment:
lm386n kit-10.jpg
lm386n kit-10.jpg [ 68.82 KiB | Viewed 2565 times ]

Larry

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 1:51 am 
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Sandy wrote:
A bit late to the party, sorry. Pardon my ignorance but can somebody tell me how far away from the frequency of a local station would this would need to be to avoid problems. I have a powerhouse at 980.
thanks,
Sandy


Hi Sandy

I have a 5Kw station at 1010 approx 10 miles away that isn't a issue... Unless the 980 is over powering the front end of your radios, I'd think you'd be OK... This little guy isn't exactly a wimp, should be able to hold his own at close range... Using only 10ft antenna(somewhat of a novelty for me), mine will cove all the sky-wave at night from 125ft...

As mentioned 1228 is also a choice, as long as you aren't setting under a 1230 it's a good option...

The 330uh coil should be OK at either frequency... A 220uh will require a fairly large capacitance to tune the antenna even at 1228, 270-300uh is a better choice ...

Tom


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 2:11 am 
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rocketeer wrote:
Anything jump out at anyone?

Larry


Hi Larry

Glad to see another build, nice job...

Unfortunately hum is as old as AM broadcasting, often the signal gets into the AC wiring and creates hum, in that instance careful location of the antenna will help... I have a 97% hum free location in my den with the antenna draped across a double window curtain rod... Hum is never noticeable unless the gain(modulation) is set far too low...

A AC cap across the line will sometimes help, use that trick in my garage... I currently have one in a AC plug I can quickly try, but have seen it work better with the cap between neutral and ground...

Another possibility is the P/S, many wall warts only have rectifiers and no filtering(is the case with mine)... Try a 220-470mfd on the B+ line... I source mine from junk TV PC boards that have a wealth of these...

On antenna tuning, one end of the cap is also a common point for the tuning screw, this end should be at ground for easiest tuning... Otherwise you need a non metallic tool to set peak...

Tom


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 2:35 am 
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35Z5 wrote:
Sandy wrote:
A bit late to the party, sorry. Pardon my ignorance but can somebody tell me how far away from the frequency of a local station would this would need to be to avoid problems. I have a powerhouse at 980.
thanks,
Sandy


Hi Sandy

I have a 5Kw station at 1010 approx 10 miles away that isn't a issue... Unless the 980 is over powering the front end of your radios, I'd think you'd be OK... This little guy isn't exactly a wimp, should be able to hold his own at close range... Using only 10ft antenna(somewhat of a novelty for me), mine will cove all the sky-wave at night from 125ft...

As mentioned 1228 is also a choice, as long as you aren't setting under a 1230 it's a good option...

The 330uh coil should be OK at either frequency... A 220uh will require a fairly large capacitance to tune the antenna even at 1228, 270-300uh is a better choice ...

Tom
I wasn't sure what your stray capacitances were so the 220 uH is almost exactly the ratio for a 'no change' to the trimmer setting to get 1.2288 MHz. I.E. 330 uH at 1 MHz must have 76.76 pF total capacitance so reverse that with 76.76 pF at 1.2288 MHz and you get 218 uH (rounded). That is, assuming the coil self capacitance is the same and I didn't screw up somewhere :wink:


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 2:39 am 
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Yup those wall warts are notorious for hum unless you get the wall warts that Radio Shack used to sell which had a transformer, rectifier, filtering and regulator all inside the case. Used to be the only wall warts I would use. Now all Radio Shack sells are the switch mode wall warts :cry:


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 3:13 am 
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Flipperhome wrote:
I wasn't sure what your stray capacitances were so the 220 uH is almost exactly the ratio for a 'no change' to the trimmer setting to get 1.2288 MHz. I.E. 330 uH at 1 MHz must have 76.76 pF total capacitance so reverse that with 76.76 pF at 1.2288 MHz and you get 218 uH (rounded). That is, assuming the coil self capacitance is the same and I didn't screw up somewhere :wink:


In the real world, I've found approx 35-40pf is needed at 1000 to tune the 330uh coil to 10ft antenna, as antenna length increases required capacitance becomes less... Of course the 1228 will require less, but I have tuned the output on my signal gen into the 330uh using a small amount of cap...

BTW with 15v(30v PP) output from my sig gen into the 330uh, it smoked when peaked... I've toasted a coil in testing, had to drop down to the 4.75v output...

Tom


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 3:21 am 
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Tube Radio wrote:

Would it be possible to use a panel mount milliammeter in place of the LED if one is not concerned about enclosure size?


Oops, missed this one...

Yes a milliammeter works fine and is the method I used when powering the Tx from my regulated B&K power supply...

I have a satellite tuning meter I've been eying for project material, has meter, LEDs, on/off sw, gain pot, would be perfect...

Tom


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 4:03 am 
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Thanks to all for the replies. I will be broadcasting to my National 183D which should be pretty good in the front end. I'll use whatever crystal I can get locally and try it. Results (and questions) to follow when completed.
Sandy


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 5:00 am 
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35Z5 wrote:
In the real world, I've found approx 35-40pf is needed at 1000 to tune the 330uh coil to 10ft antenna, as antenna length increases required capacitance becomes less... Of course the 1228 will require less, but I have tuned the output on my signal gen into the 330uh using a small amount of cap...
That number comports with my experience because, if I remember correctly, I measured my 10 foot wet noodle at around 35 pF so toss in a few pF for stray and between 35 pF and 40 pF brings it in.

35Z5 wrote:
BTW with 15v(30v PP) output from my sig gen into the 330uh, it smoked when peaked... I've toasted a coil in testing, had to drop down to the 4.75v output...
Hmm. I know the circulating currents are significant but I'm surprised it actually fried. What's the power rating on those little things?


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 9:38 pm 
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Tube Radio wrote:
Yup those wall warts are notorious for hum unless you get the wall warts that Radio Shack used to sell which had a transformer, rectifier, filtering and regulator all inside the case. Used to be the only wall warts I would use. Now all Radio Shack sells are the switch mode wall warts :cry:

Is this it? Picked this up at a flea market this morning. The plug is unique in that the hole for the pin is smaller than most other 9v supplys. The image on the front brings up a question. Not being familiar with the polarity of the plugs for these wall warts I assumed the center was "+" and the outer was "-" but if I'm reading the illustration correctly it's just the opposite? I just checked out nine other supplys i have and none of them have that illustration.
Attachment:
rs ps.jpg
rs ps.jpg [ 76.38 KiB | Viewed 2532 times ]

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 10:24 pm 
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Yeah the center pin is negative on that one, most have the center as positive... I usually just chop off the connector and connect them direct...

Would be a good idea to see what the unloaded output voltage measures... Rated for a 300ma load, I'm betting it's closer to 16v, which is too much for this transmitter... The wallwort I have on the breadboard is a 3.7v @ 350ma and delivers approx 11.5v, so I use a 100 ohm and the LED to drop to 8.7v...

Remember this... At 11ma (what tx draws with antenna in tune), each 10 ohms in series will drop .11v... If you need to reduce voltage by 3v, you'd need approx 270 ohms...

Tom


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 04, 2012 11:05 pm 
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35Z5 wrote:
Yeah the center pin is negative on that one, most have the center as positive... I usually just chop off the connector and connect them direct...

Would be a good idea to see what the unloaded output voltage measures... Rated for a 300ma load, I'm betting it's closer to 16v, which is too much for this transmitter... The wallwort I have on the breadboard is a 3.7v @ 350ma and delivers approx 11.5v, so I use a 100 ohm and the LED to drop to 8.7v...

Remember this... At 11ma (what tx draws with antenna in tune), each 10 ohms in series will drop .11v... If you need to reduce voltage by 3v, you'd need approx 270 ohms...

Tom

The one pictured above is putting out 11v but the other 9v 300ma I have is putting out 16V. Yikes!

Larry

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Thu 05, 2012 12:29 am 
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Well the old saying is "a picture is worth a 1000 words", so here is the schematic of my breadbord version operating on a wallwort supply...

http://amradio.freeiz.com/transmitter/386txps.jpg

Remember voltage output will vary greatly by supply, so the value of the 100 ohm resistor will need to be calculated...


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Thu 05, 2012 1:26 am 
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Problem solved. I found a Radio Shack 9 volt battery eliminator power supply. It put out 9.4 volts. I wired it direct and then recalibrated everything. This thing is great! Clear as a bell and great range. Thanks to everyone who helped. :D

Attachment:
lm386n kit-11.jpg
lm386n kit-11.jpg [ 42.9 KiB | Viewed 2515 times ]

Larry

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Fri 06, 2012 1:27 am 
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35Z5 wrote:

I have a satellite tuning meter I've been eying for project material, has meter, LEDs, on/off sw, gain pot, would be perfect...

Tom


Nice


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 11, 2012 4:11 am 
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I did a little comparison of this LM386 Tx to a SStrans, again I used the camera to record audio, but a major difference would be easy to detect...

Clip begins with SStrans, switches @ :18 to LM386, back to SS @ :43, again to LM386 @1:00 and finally back to SS @1:19... You'll have to listen closely for the switch to the LM386, but it's fairly obvious when the LM386 is shutdown, as it creates a good bit of noise.... The switch was made by powering up/down each transmitter, SStrans was also tuned to 1000Khz...

http://amradio.freeiz.com/transmitter/txcomp.mp3


Tom


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Wed 11, 2012 4:31 pm 
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35Z5 wrote:
Sounds impressive enough eh? Well for the parts count(11)it is, I've spent more time in development with this little guy than most of the tube units... Sound quality is very good and the current drain at 9v is only 11ma, so can be easily powered by a std 9v battery... The coil coupling the audio to the osc is very important, without it current draw is increased by 60%, is essential for battery life... The LM386L I first worked with howled terribly without the coil in place but the "N" version performs approx same with or without...
Image

Schematic drawn to approximate layout on the R/S 276-148 PCB... Optional components not installed, not needed unless one is using a non std line source...

Image

Bottom, was prettier till I had to reinstall the ground network, BTW all solder connections were made with a Weller 8200...

Image

I was running short on 10K resistors so subbed 8.2K in the input network with no issues, most of the prototype work was performed using a pair of 4.7K so any value between 4.7 & 10K should be fine...

Image

I was going to use a small R/S project box but found this Honeywell HVAC tester that had "modulate" on the dials so couldn't resist... Antenna will likely be moved to top of case and connector replaced with indicator LED...

Image

Tom, as a newbie, I don't understand the coil. What part is it in the pictures?
Thanks, Rickey

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